Static spawns are exactly that, static. Same stuff everywhere no matter what. There’s supposedly SOME dynamic behavior (making a ton of noise can add more zombies, I’m told, but I’ve never experienced it).
If you walked through a city, even stealthily, you will see a population of zombies. When you kill them all, that’s it. The city is yours now.
Dynamic spawns react to player behavior and time to throw enemies at you. If you sneakily wander into a city you wont see hardly anything around. The instant you smash a window though, enjoy your new fanclub. Supposedly you wont see the really hardcore stuff until some time has passed in the game (don’t want to mention anything specific for spoilers) but I can’t confirm this either. I know that if you fire off a shotgun in the middle of the city you’re going to see some pretty angry special zombies.
The key difference is that Dynamic spawns give you a never ending stream of undead. You can’t ever really clear out an area, because the game will keep sprinkling enemies around and any noise you make inside a city will cause a wave to spawn.
Strategy wise, Static tends more towards “take and hold” combat heavy gameplay and becoming more invested in an area…building up a safehouse in the city.
Dynamic is more stealthy, and has you doing quick raids on areas to get supplies and then get out. You can still build a stronghold but you’re probably looking to build a nice cabin up in the woods somewhere vs. finding an apartment to rent from a Zombie Hulk.